The invention relates to a method for monitoring the operations of an electromechanical control system that ensures the adhesion of wheels of a motor vehicle and to a safety switching arrangement for carrying out the indicated method.
It is generally known that the components of control systems of this type, such as the ABS (antilock brake systems) or the ASR (wheel slip control system), that are relevant to driving safety, must be monitored with respect to their correct functioning. From DE-PS No. 31 27 301, a safety switching arrangement is known to which certain input signals are fed and that evaluates these input signals by predetermined operating criteria, particularly the plausibility of the simultaneous occurrence of different input signals. If signals are present simultaneously that, if the ASR or ABS control system functions properly, should not occur simultaneously, the safety switching arrangement, by an error switching signal, interrupts the signal flow of the control system and warns the driver of the vehicle by switching on a control light.
It is known to use, in combination with wheel slip and antilock control systems, acceleration sensors that are fixed at the vehicle for forming certain reference signals, such as the longitudinal acceleration of the vehicle and the driving speed. This is shown in DE-OS No. 35 45 652 and corresponding U.S. patent application No. 943,693, filed Dec. 19, 1986. They are required mainly in order to be able to obtain, in the case of four-wheel driven vehicles equipped with ASR/ABS, at any time, references values for slip comparators that are independent of the slip conditions between the wheels and the ground, and they are therefore also relevant with respect to driving safety.
It is also known to use acceleration sensors in vehicles for the triggering control of passive safety systems, such as air bags and belt tighteners.
It is the objective of the invention to provide a process for the monitoring of operations of an electromechanical control system of the described type that permits the monitoring of an acceleration sensor in the motor vehicle that corresponds with a control system of this type as well as to develop the known safety switching arrangement such that it can be used for implementing the indicated process.
The proper functioning of a control system (such as an ABS, ASR) that secures the adhesion of the wheels, by analyzing wheel rotating speed signals, is determined by comparing acceleration signals determined from the wheel rotating speed signals with accelerated signals measured by acceleration sensors on the vehicle. If they do not correspond within given limits during absence of control system control signal, the control system is disabled by an error signal and a warning indicator is activated.
In principle, antilock and wheel slip control systems have wheel rotating speed sensors with signals converters. From their wheel rotating speed signals, the vehicle speed and acceleration can be determined as reference signals in known ways with some precision as long as only the unavoidable rolling slip is present at the wheels. However, as soon as the ABS or the ASR intervene, decelerations and accelerations occur at the slip-controlled wheels that change at a high frequency. Thus, a clear relationship can no longer be established between the wheel rotating speed signals and the vehicle speed or the vehicle acceleration, but can be represented only approximately by standard values.
The invention is based on the recognition that the signals of an acceleration sensor, that is fixed at the vehicle and that is used mainly for obtaining reference signals that are independent of the wheel slip, can be monitored by comparison with vehicle acceleration signals converted or calculated from wheel rotating speed signals, as long as the assigned control system does not intervene.
The blocking intervention of the safety switching arrangement into the signal flow of the control system, according to the invention, is prevented for two reasons when the control system at the time is in an operating cycle:
on the one hand, the control system that is operating at the time, for understandable reasons, must not be switched off "for reasons of safety"; PA1 on the other hand, the acceleration signals that are determined from the wheel rotating speeds and are measured by the acceleration sensor can then not be within the range of the conventional consistent band width, as indicated above.
The safety switching arrangement permits the simultaneous monitoring of the sensor acceleration signals that are assigned to the three coordinate axes of the vehicle, the longitudinal (x-) axis, the transverse (y-) axis and the vertical (z-) axis. The longitudinal and transverse accelerations of the vehicle are already included in the signal flow of the ABS and the ASR in a known way. The additional detection of vertical accelerations of the vehicle body (that is also know per se) becomes interesting, for example, when a vehicle is also equipped with an automatic spring-damper control system which also ensures the adhesion of the vehicle wheels. In the latter case, according to the invention, the vertical parts of the wheel acceleration signals must be obtained from the wheel acceleration signals by filtering out all other parts and can then be compared with vertical acceleration signals of the acceleration sensor that is fixed at the vehicle, naturally only while taking into account the transmission function of the spring-damper system.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.